1. Field of the Invention
The present invention relates to a localization method of multiple jammers based on a TDOA method, and which can accurately estimate locations of multiple jammers using TDOA measurements of jamming signal between reference sensor and other sensors.
2. Description of Related Art
GPS (Global Positioning System) is a system for providing services like time, position and velocity of an object using satellites. GPS positioning is obtained by using the received GPS signals at a GPS receiver, and the GPS signals are transmitted from the satellites in the space.
Since GPS signals are transmitted from GPS satellites located at an altitude of about 20,000 km, the power of GPS signals received on the ground is weaker than noise power. In order to receive such weak signal, a GPS receiver should have high sensitivity. As a result, since the GPS receiver receives weak interference signal generated from other signal source and/or even jamming signal, it is very difficult or impossible to acquire good GPS signals and perform the localization. Therefore, in case that the interference signals and/or jamming signals are included in the GPS signals, availability of the GPS is extremely deteriorated.
Recently, damage from the jamming signal, such as aircraft navigation problems, has been sharply increased, and thus a method of reducing the damage from such jamming signal has been studied. As examples of the localization method of the jammer, there are a TOA (Time Of Arrival) method using a signal arrival time, a TDOA (Time Difference Of Arrival) method using a time difference of signal arrival, an AOA (Angle Of Arrival) method using an arrival angle of signal, and an RSSI (Received Signal Strength Indication) method using a signal strength.
However, in case of the TOA method, time synchronization between transmitter and receiver is required. In case of the RSSI method, it has low accuracy. In case of the AOA method, it has a lower positioning accuracy than the TDOA method. Therefore, TDOA method has become as a typical method of localizing the source of the jammer.
Localization based on a TDOA method uses the time difference of signal arrival, as shown in FIG. 1a, and the location of the jammer is calculated by using a time difference of signal arrival between a reference sensor and other sensors. Herein, the location of the jammer calculated by using TDOA measurement can be represented in the form of a hyperbolic curve, and the position of the jammer can be determined by finding an intersection point of the hyperbolic curves.
Herein, the time difference of signal arrival between the two sensors can be calculated by using a cross-correlation function indicated by Equation 1. When the signals received in the two sensors are cross-correlated with each other, a cross-correlation value Rri(τ) forms a curve shown in FIG. 1. 1b and has one maximum value. A delay time in the maximum value is the time difference rri of arrival, i.e., TDOA measurement value.
                                                        R              ri                        ⁡                          (              τ              )                                ≡                      E            ⁡                          [                                                                    s                    r                                    ⁡                                      (                    t                    )                                                  ⁢                                                      s                    i                                    ⁡                                      (                                          t                      -                      τ                                        )                                                              ]                                      =                              1            T                    ⁢                                    ∫              0              T                        ⁢                                                            s                  r                                ⁡                                  (                  t                  )                                            ⁢                                                s                  i                                ⁡                                  (                                      t                    -                    τ                                    )                                            ⁢                                                          ⁢                              ⅆ                τ                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
Wherein sr(t) is signal received at reference sensor, si(t) is signal received at ith sensor, T is the period of signal, τ is time delay.
However, in case that there are two or more jamming signals, multiple maximum values (peaks) are formed when performing the cross-correlation with respect to the received signals. In this case, since it is difficult to distinguish true values and false values among the maximum values, it is actually impossible to localize the multiple jammers by using only the conventional TDOA method.
Therefore, it is required to develop a new localization method of multiple jammers, which can accurately localize the jammers even when there are multiple jammers.